Optoelectronic devices based on atomically thin semiconductors and photo-oxidised HfOx

The direction of research in solid state physics and technology has changed since the discovery of graphene. Now, a plethora of two-dimensional materials are being thoroughly investigated for their unique properties as well as for their implementation in next-generation optoelectronic devices. Of course, much effort is needed in order to reach the current level of modern electronics which is based on decades of research and development. For example, the level of miniaturisation modern technology requires can be achieved with atomically thin materials, driving Moore's Law forward. Conventional dielectrics exhibit high leakage currents when their dimensions are reduced to the nano-scale and the need for alternative materials compatible with two-dimensional electronics arises. However, the techniques that are being used for the growth and processing of conventional semiconducting materials are not always suitable with two-dimensional materials, which need special handling. These are some of the points that will be addressed in this PhD dissertation. Here, a new method for generating a fundamentally two-dimensional high-k dielectric which can be automatically incorporated in atomically thin optoelectronics devices is presented. The photo-oxidation of hafnium disulfide, HfS2, is a straight-forward, non-invasive process that can be used to oxidise pristine few-layered HfS₂, opening new paths for applications ranging from optoelectronics to photonics. The resulting dielectric, Hafnium dioxide, HfO₂, exhibits outstanding properties that exceed those of silicon dioxide, SiO₂ and its atomically thin nature makes it an ideal insulating layer for next-generation nano-electronics. Finally, the last part of this thesis is dedicated to a novel, CVD-grown, n-type monolayer of tungsten diselenide, WSe2. This is the first time negatively doped CVD-grown WSe₂ is reported, which opens the possibility of choosing the doping of the two-dimensional semiconductor before fabrication. For investigating and characterising this novel material, field-effect transistors are fabricated and characterised optoelectronically, shining light on the carriers' behaviour and the ability of the material in light-detection applications. Vacuum and ambient annealing of the WSe2 based devices highlights a possible way to control the doping level of the material, and thus the electrical behaviour of the devices.Engineering and Physical Sciences Research Council (EPSRC

An Investigation of the Behavior of Raw Calcareous Fly Ash in Mortar Mixtures

The behavior of unprocessed calcareous fly ash in cement mortar mixtures is often questionable and unpredictable and that is one of the difficulties in confining these fly ashes within the limits of standards. This work aims at correlating the material characteristics with its behavior in mortar mixtures. An effort is made to understand how three of the reactive constituents; lime (CaO), silica (SiO2), and sulfates (SO3), affect strength and volume stability. Two raw calcareous fly ashes of different composition emanating from the Ptolemaida area, Greece, were systematically tested to define their profile as cementitious agents that could be incorporated in a cement mixture by replacing part of the cement. Apart from chemical analysis, a mineralogical analysis of their phases was carried out by x-ray diffraction and SEM microscopy. Fineness was measured by sieving and laser particle size analysis to identify the most prevalent grain size. In order to measure soundness and water demand of the pastes, the corresponding EN and ASTM standards were followed. A number of specimens were also prepared in order to determine the pozzolanicity index of the fly ashes. Furthermore, mortar mixtures with either calcareous fly ashes as the only binder or in combination with cement (50/50) were prepared both with and without superplasticizers (SPL) in order to monitor strength development. The early deformation of these mortars, stored at low relative humidity (less than 65%) and a temperature of 20 uC, was also measured. Evaluating all the results, some remarks are made about the performance of the existing standards. It is also obvious that the water/binder ratio is an important parameter which can be modified by using SPL and in this way it is possible to improve significantly strength and volume stability of cement-fly ash mortar mixtures

NNLO phase space master integrals for two-to-one inclusive cross sections in dimensional regularization

We evaluate all phase space master integrals which are required for the total cross section of generic 2 -> 1 processes at NNLO as a series expansion in the dimensional regulator epsilon. Away from the limit of threshold production, our expansion includes one order higher than what has been available in the literature. At threshold, we provide expressions which are valid to all orders in terms of Gamma functions and hypergeometric functions. These results are a necessary ingredient for the renormalization and mass factorization of singularities in 2 -> 1 inclusive cross sections at NNNLO in QCD.Comment: 37 pages, plus 3 ancillary files containing analytic expressions in Maple forma

Subtraction at NNLO

We propose a framework for the implementation of a subtraction formalism at NNLO in QCD, based on an observable- and process-independent cancellation of infrared singularities. As a first simple application, we present the calculation of the contribution to the e+e- dijet cross section proportional to C_F T_RComment: 42 pages Latex; 7 figures included. Modifications to the text, and references added; the results are unchange

The fully differential hadronic production of a Higgs boson via bottom quark fusion at NNLO

The fully differential computation of the hadronic production cross section of a Higgs boson via bottom quarks is presented at NNLO in QCD. Several differential distributions with their corresponding scale uncertainties are presented for the 8 TeV LHC. This is the first application of the method of non-linear mappings for NNLO differential calculations at hadron colliders.Comment: 27 pages, 13 figures, 1 lego plo

Hepta-Cuts of Two-Loop Scattering Amplitudes

We present a method for the computation of hepta-cuts of two loop scattering amplitudes. Four dimensional unitarity cuts are used to factorise the integrand onto the product of six tree-level amplitudes evaluated at complex momentum values. Using Gram matrix constraints we derive a general parameterisation of the integrand which can be computed using polynomial fitting techniques. The resulting expression is further reduced to master integrals using conventional integration by parts methods. We consider both planar and non-planar topologies for 2 to 2 scattering processes and apply the method to compute hepta-cut contributions to gluon-gluon scattering in Yang-Mills theory with adjoint fermions and scalars.Comment: 37 pages, 6 figures. version 2 : minor updates, published versio

Improving the sensitivity of Higgs boson searches in the golden channel

Leptonic decays of the Higgs boson in the ZZ* channel yield what is known as the golden channel due to its clean signature and good total invariant mass resolution. In addition, the full kinematic distribution of the decay products can be reconstructed, which, nonetheless, is not taken into account in traditional search strategy relying only on measurements of the total invariant mass. In this work we implement a type of multivariate analysis known as the matrix element method, which exploits differences in the full production and decay matrix elements between the Higgs boson and the dominant irreducible background from q bar{q} -> ZZ*. Analytic expressions of the differential distributions for both the signal and the background are also presented. We perform a study for the Large Hadron Collider at sqrt{s}=7 TeV for Higgs masses between 175 and 350 GeV. We find that, with an integrated luminosity of 2.5 fb^-1 or higher, improvements in the order of 10 - 20 % could be obtained for both discovery significance and exclusion limits in the high mass region, where the differences in the angular correlations between signal and background are most pronounced.Comment: 31 pages, 8 figures. v2: Minus signs in definitions of angles corrected. Typos fixed. Reference added. Cosmetic changes to Figure 4. Additional sentence added for clarificatio

Factorization and NNLL Resummation for Higgs Production with a Jet Veto

Using methods of effective field theory, we derive the first all-order factorization theorem for the Higgs-boson production cross section with a jet veto, imposed by means of a standard sequential recombination jet algorithm. Like in the case of small-q_T resummation in Drell-Yan and Higgs production, the factorization is affected by a collinear anomaly. Our analysis provides the basis for a systematic resummation of large logarithms log(m_H/p_T^veto) beyond leading-logarithmic order. Specifically, we present predictions for the resummed jet-veto cross section and efficiency at next-to-next-to-leading logarithmic order. Our results have important implications for Higgs-boson searches at the LHC, where a jet veto is required to suppress background events.Comment: 28 pages, 5 figures; v2: published version; note added in proo

On the Numerical Evaluation of Loop Integrals With Mellin-Barnes Representations

An improved method is presented for the numerical evaluation of multi-loop integrals in dimensional regularization. The technique is based on Mellin-Barnes representations, which have been used earlier to develop algorithms for the extraction of ultraviolet and infrared divergencies. The coefficients of these singularities and the non-singular part can be integrated numerically. However, the numerical integration often does not converge for diagrams with massive propagators and physical branch cuts. In this work, several steps are proposed which substantially improve the behavior of the numerical integrals. The efficacy of the method is demonstrated by calculating several two-loop examples, some of which have not been known before.Comment: 13 pp. LaTe

Second order QCD corrections to inclusive semileptonic b \to Xc l \bar \nu_l decays with massless and massive lepton

We extend previous computations of the second order QCD corrections to semileptonic b \to c inclusive transitions, to the case where the charged lepton in the final state is massive. This allows accurate description of b \to c \tau \bar \nu_\tau decays. We review techniques used in the computation of O(\alpha_s^2) corrections to inclusive semileptonic b \to c transitions and present extensive numerical studies of O(\alpha_s^2) QCD corrections to b \to c l \bar \nu_l decays, for l =e, \tau.Comment: 30 pages, 4 figures, 5 table